In recent years, there has been strong demand for a non-volatile memory that operates at high speed with little power consumption and that is compact but still has a large storage capacity, because there is an increased need for processing high-volume image information even in such devices as portable telephones and personal digital assistants (PDAs). A ferroelectric memory disclosed, for example, in the specification of USP Published Application No. 2002/017675 (Japanese Unexamined Patent Publication No. 2001-298162) is a known non-volatile memory. Recently, a memory that uses the property that the resistance value of a bulk varies depending on the condition of its crystal (i.e., so-called phase-change memory) has received widespread attention as a memory that is both super-integrated and capable of non-volatile operation. Phase-change memories are disclosed, for example, in the specification of U.S. Pat. No. 5,296,716 and WO97/40499A1 (Japanese Unexamined Patent Publication No. 2000-509204).
A phase-change memory is structured so as to hold a phase-change film that comprises a plurality of chalcogens between two electrodes, wherein Joule's heat is applied to the phase-change film by applying current across the two electrodes to change the condition of crystals between non-crystalline and crystalline phases, thereby performing electrical switching. For example, in GeSbTe-based phase-change materials and the like, a plurality of crystalline phases coexist, and, theoretically, it is possible to change the resistance value between the two electrodes in an analog fashion. Therefore, GeSbTe base phase-change materials are anticipated to be useful as analog (multivalued) memories as well as memories for digital circuits. Because the crystalline condition of a memory activity region is extremely stable at room temperature, it is assumed that it is possible to sufficiently retain memory over 10 years.
In such a phase-change memory, when the crystalline condition is changed from a low resistance condition to a high resistance condition, the temperature of the memory active region is locally raised to a high temperature and the temperature is expected to reach approximately 600° C. However, when a device is fabricated using a phase-change film formed of GeSbTe, for example, by sandwiching it between aluminum thin films, which are generally used in LSI fabricating processes, mutual reaction between the phase-change film and the aluminum film occurs by heat treatment, even at 100° C., significantly impairing the performance of the device. Therefore, high thermal stability is needed in the material for the electrodes that sandwich a phase-change material in between and prevention of impurity diffusion is required to avoid the elements composing the electrodes from mixing into the phase-change material.
WO97/05665A1 (National publication of the translated version of PCT application No. 1999-510317) discloses a structure wherein a thin film contact layer containing at least one element selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W and two or more members selected from the group consisting of B, C, N, O, Al, Si, P, S, and their mixtures or alloys, is provided between a phase-change film and an electrode. However, even in a phase-change memory having such compositions, since it is difficult to achieve satisfactory heat resistance and prevention of impurity diffusion, there results the problem that the possible number of rewrite cycles is not enough to use it as an alternative to widely used general-purpose memories such as DRAMs (dynamic random access memories) and SRAMs (static random access memories).